Combine all dry ingredients (coconut flour, baking powder, unsweetened cocoa powder, salt and sweetener) in a mixing bowl. Set aside.
In a small bowl, microwave butter for a few seconds to melt.
To the bowl with melted butter, add the rest of the wet ingredients (egg, heavy cream, and vanilla extract) and combine with the bowl of dry ingredients.
Pour batter into a large microwave-safe mug.
Microwave on high about 1 minute. Most of the top should look done.
Let cake rest in mug for 1 minute. Run a knife around the cake inside the mug to loosen if transferring to a plate, or eat straight from the mug with a spoon.
Serve with the toppings of your choice and enjoy!